The Client-Server Mode 客户-服务器模型
The term server applies to any program that offers a service that can be reached over a network. A server accepts a request over the network, performs its service, and returns the result to the requester. For the simplest services, each request arrives in a single IP datagram and the server returns a response in another datagram.
An executing program becomes a client when it sends a request to a server and waits for a response. Because the client-server model is a convenient and natural extension of interprocess communication on a single machine, it is easy to build programs that use the model to interact.
Servers can perform simple or complex tasks. For example, a time-of-day server
merely returns the current time whenever a client sends the server a packet. A web server receives requests from a browser to fetch a copy of a Web page; the server obtains a copy of the file for the page and returns it to the browser.
Usually, servers are implemented as application programs. The advantage of implementing servers as application programs is that they can execute on any computing system that supports TCP/IP communication. Thus, the server for a particular service can execute on a timesharing system along with other programs, or it can execute on a personal computer. Multiple servers can offer the same service, and can execute on the same machine or on multiple machines. In fact, managers commonly replicate copies of a given server onto physically independent machines to increase reliability or improve performance. If a computer's primary purpose is support of a particular server program, the term "server" may be applied to the computer as well as to the server program.
Thus, one hears statements such as "machine A is our file server."
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